In order to enhance the volumetric efficiency of modern internal combustion engines, whether of the spark ignition type or the compression ignition (diesel) type, forced induction devices, such as turbochargers and superchargers are added to the engine. Furthermore, most modern internal combustion engines utilize an exhaust gas recirculation (EGR) system in which a portion of an engine's exhaust gas is recirculated back to the engine cylinders. EGR is used mainly to reduce nitrogen oxide (NOx) emissions. By mixing the incoming air with recirculated exhaust gas, the fluid mixture is diluted with inert gas (the recirculated gas), lowering the flame temperature and reducing the amount of excess oxygen in diesel engines. The exhaust gas also increases the specific heat capacity of the mix, lowering the peak combustion temperature, thereby limiting the formation of NOx.
An external EGR system routes exhaust gas from the exhaust manifold to the inlet manifold. A control valve (EGR Valve) within the system regulates and times the exhaust gas flow. Some engine designs trap exhaust gas within the cylinder by not fully expelling it during the exhaust stroke. This is referred to as an internal EGR system.